1. Field
The field of the invention is related to battery charging devices, and more particularly to monolithically formed battery charging devices having at least one voltage step-down direct-current-to-direct-current converter.
2. Related Art
In general, direct-current-to-direct-current (xe2x80x9cDCxe2x80x94DCxe2x80x9d) converters fall into two categories, namely step-up converters and step-down converters. As the category names imply, a step-up DCxe2x80x94DC converter provides an output voltage that is stepped up from (i.e., greater than) an applied input voltage, and a step-down DCxe2x80x94DC converter provides an output voltage that is stepped down from (i.e., less than) an applied input voltage. Because of the conservation of energy and physical dissipation losses, the current that is output from the DCxe2x80x94DC converter must be less than the input current in the case of step-up converter.
Because the voltage output in a step-down DCxe2x80x94DC converter is less than the input voltage, the output current can be greater than the input current, keeping in mind that the input power should be approximately equal to the output power minus circuit-load losses. Given that input power and output power will be approximately equal, then the circuit can be modeled by the by the transfer function Vout=D*Vin, wherein Vout is the output voltage, Vin is the input voltage, and D is the duty ratio (also known as the step-down ratio); or by the transfer function Iout=Iin/D, wherein Iout is the output current, and Iin is the input current. Thus, the duty ratio of the output voltage to the input voltage is approximately equal to the ratio of the input current to the output current.
The duty ratio is useful in designing devices that employ step-down DCxe2x80x94DC converters. In practice, step-down DCxe2x80x94DC converters are able to power devices with various voltages and corresponding current requirements by employing the duty ratio designed into the DCxe2x80x94DC converter. For instance, a step-down DCxe2x80x94DC converter can provide enough drive current to charge a battery even though the input current may be insufficient to charge the battery load.
Existing step-down DCxe2x80x94DC converters normally include an internal oscillator to convert or chop a direct-current (DC) input supply signal into alternating current (AC) signal. After conversion, the DCxe2x80x94DC converter rectifies and filters the AC signal to provide a final desired DC voltage. The oscillator used in some prior art DCxe2x80x94DC converters is a free running type that operates at a constant frequency. To minimize dissipation of energy in such circuits, driver circuitry is connected to the oscillator to convert or otherwise chop the DC input supply signal using pulse width modulation (PWM) techniques into a series of pulses or a pulse train. The width of each pulse in the pulse train may be determined by the desired output of the DCxe2x80x94DC converter. The switching frequency is normally chosen to optimize switching efficiency and the gain-phase characteristics of filtering devices.
Such prior art DCxe2x80x94DC converters typically include a switch that connects a high-side output of an external DC input supply to an external series inductor, that in turn is connected to an external series capacitor, which is coupled to the low-side output of the DC input supply. The DC input supply provides a signal to drive the combination of the external inductor and capacitor so that a desired voltage and current combination may be supplied between an output node, i.e., the node connecting the capacitor and the inductor, and the low-side output node of the DC input supply. Generally, the inductor and capacitor combination are selected to provide a desired output voltage level that corresponds to the duty cycle of the switch.
Additionally, prior art DCxe2x80x94DC converters have control circuitry to control the duty cycle of the switch. In these prior art DCxe2x80x94DC converters, this control circuit adjusts the duty cycle of the switch by sensing the output voltage across the capacitor and then adjusting PWM of the switch. Due to operating inefficiency in the duty cycle of the switch, among other things, prior art DCxe2x80x94DC converters use one or more external transformers to achieve large duty ratios, which limit the design and manufacture of monolithic the DCxe2x80x94DC converters having large duty ratios.
Ultimately, these transformers may prevent the design and manufacture of monolithic DCxe2x80x94DC converters having large duty ratios or dramatically increase the package size of the DCxe2x80x94DC converters making manufacture cost unacceptable. In addition, as noted above, prior art step-down DCxe2x80x94DC converters suffer from having to employ external inductors and capacitors due to many factors, including the resonant frequency of the inductors, capacitor, and battery load.
To overcome some of these limitations, some manufacturers have resorted to monolithic DCxe2x80x94DC converters. Known monolithic DCxe2x80x94DC converters, however, suffer from high inductor current, inductor saturation and switch saturation, which result in low efficiency and small duty ratios. Another known step-down DCxe2x80x94DC converter includes a resonant gate drive for very low voltage applications, but the circuit topology does not use the current feedback, which is generally required in a battery charger application.
Therefore, what is needed is an efficient, monolithically-formed-step-down DCxe2x80x94DC converter that can supply enough drive current to charge a battery without inductor and switch saturation. Further, such monolithically formed step-down DCxe2x80x94DC converter should provide small as well as large duty ratios.
In an exemplary embodiment, a monolithic battery charger includes a step-down converter having a duty ratio in the range of approximately 10 to 95 percent. Each of the step-down converters may be formed from a monolithically formed buck-type regulator coupled to or integrated with at least one monolithically or discretely formed capacitor, and a monolithically or discretely formed inductor in a standard buck configuration. Alternatively, the at least one monolithically or discretely formed capacitor, and the monolithically or discretely formed inductor may be integral to the monolithically formed buck-type regulator.
Each of the monolithically formed buck-type regulators may include a monolithic controller, a monolithic switch, and a monolithic rectifier. Unlike a standard buck configuration, however, the controller operates at a switching frequency of at least one megahertz (MHz). In addition to the step-down converter, the monolithic battery charger includes a battery-terminal interface connected to the step-down converter. This battery-terminal interface provides an output voltage and current that may be used to recharge a rechargeable battery. In one embodiment, the monolithic battery charger may be directly incorporated into a rechargeable battery.
In another exemplary embodiment, a monolithic battery charger includes at least one step-down converter that has a duty ratio in the range of approximately 10 to 90 percent. Each of the step-down converters may be formed from a plurality of cascaded (serially connected) monolithically formed buck-type regulators, as noted above. Each of these serially connected monolithically formed buck-type regulators has a duty ratio of in the range of approximately 10 to 95 percent. And the controller in each of the serially connected monolithically formed buck-type regulators operates at a switching frequency of at least one MHz.